1. Field of the Invention
This invention is in the field of cooling biological tissues to very low temperatures, for treatment of medical conditions, as in cryosurgery.
2. Background Information
It is desirable to be able to selectively cool miniature discrete portions of biological tissue to very low temperatures in the performance of cryosurgery, without substantially cooling adjacent tissues of the organ. Cryosurgery has become an important procedure in medical, dental, and veterinary fields. Particular success has been experienced in the specialties of gynecology and dermatology. Other specialties, such as neurosurgery and urology, could also benefit from the implementation of cryosurgical techniques, but this has only occurred in a limited way. Unfortunately, currently known cryosurgical instruments have several limitations which make their use difficult or impossible in some such fields. Specifically, known systems are not optimally designed to have sufficient precision and flexibility to allow their widespread use endoscopically and percutaneously.
In the performance of cryosurgery, it is typical to use a cryosurgical application system designed to suitably freeze the target tissue, thereby destroying diseased or degenerated cells in the tissue. The abnormal cells to be destroyed are often surrounded by healthy tissue which must be left uninjured. The particular probe, catheter, or other applicator used in a given application is therefore designed with the optimum shape, size, and flexibility or rigidity for the application, to achieve this selective freezing of tissue. Where a probe or catheter is used, the remainder of the refrigeration system must be designed to provide adequate cooling, which involves lowering the operative portion of the probe to a desired temperature, and having sufficient power or capacity to maintain the desired temperature for a given heat load. The entire system must be designed to place the operative portion of the probe or catheter at the location of the tissue to be frozen, without having any undesirable effect on other organs or systems.
Currently known cryosurgical systems typically use liquid nitrogen or nitrous oxide as coolant fluids. Liquid nitrogen is usually either sprayed onto the tissue to be destroyed, or it is circulated to cool a probe which is applied to the tissue. Liquid nitrogen has an extremely low temperature of approximately 77 K, and a high cooling power, making it very desirable for this purpose. However, in these systems, liquid nitrogen typically evaporates and escapes to the atmosphere during use, requiring the continual replacement of storage tanks. Further, since the liquid is so cold, the probes and other equipment used for its application require vacuum jackets or other types of insulation. This makes the probes relatively complex, bulky, and rigid, and therefore unsuitable for endoscopic or intravascular use. The need for relatively bulky supply hoses and the progressive cooling of all the related components make the liquid nitrogen instruments less than comfortable for the physician, as well, and they can cause undesired tissue damage.
A nitrous oxide system typically achieves cooling by pressurizing the gas and then expanding it through a Joule-Thomson expansion element, such as a valve, orifice, or other type of flow constriction, at the end of a probe tip. The typical nitrous oxide system pressurizes the gas to 700 to 800 psia., to reach practical temperatures of no lower than about 190 K to 210 K. Nitrous oxide systems are not able to approach the temperature and power achieved by the nitrogen systems. The maximum temperature drop that can be achieved in a nitrous oxide system is to 184 K, which is the boiling point of nitrous oxide. The nitrous oxide system does have some advantages, in that the inlet high pressure gas is essentially at room temperature until it reaches the Joule-Thomson element at the probe tip. This eliminates the need for insulation of the system, facilitating miniaturization and flexibility to some extent. However, because of the relatively warm temperatures and low power, tissue destruction and other applications are limited. For many such applications, temperatures below 184 K are desirable. Further, the nitrous oxide must typically be vented to atmosphere after passing through the system, since affordable compressors suitable for achieving the high pressures required are not reliable and readily commercially available.
In most Joule-Thomson systems, single non-ideal gasses are pressurized and then expanded through a throttling component or expansion element, to produce isenthalpic cooling. The characteristics of the gas used, such as boiling point, inversion temperature, critical temperature, and critical pressure determine the starting pressure needed to reach a desired cooling temperature. Joule-Thomson systems typically use a heat exchanger to cool the incoming high pressure gas with the outgoing expanded gas, to achieve a higher drop in temperature upon expansion and greater cooling power. For a given Joule-Thomson system, the desired cooling dictates the required heat exchanger capacity.
Specifically, it would be desirable to develop a refrigeration system which can apply the necessary cooling power through a long, slender, flexible catheter, such as a transvascular cardiac catheter, or a cryoprobe. Cardiac catheters must be very slender, in the range of less than 5 mm., and they must exhibit considerable flexibility, in order to be inserted from an access point in a remote blood vessel into the heart. A cryosurgical catheter to be used in such an application must also have a relatively low operating pressure for safety reasons. It must have the cooling power to overcome the ambient heat load imposed by the circulating blood, yet it must be able to achieve a sufficiently low temperature to destroy the target tissue. Finally, the cold heat transfer element must be limited to the tip or end region of the catheter, in order to prevent the damaging of tissue other than the target tissue.
It is an object of the present invention to provide a method and apparatus for precooling a primary loop high pressure refrigerant with a secondary loop refrigeration cycle which uses a different refrigerant from the primary loop, and then using the primary loop high pressure refrigerant to achieve a sufficiently low temperature, utilizing a pressure which is safe for cryosurgery, with components capable of fitting within a hand held cryoprobe or flexible intravascular catheter.
The present invention comprises a miniature refrigeration system, including a method for operating the system, including precooling of the primary high pressure refrigerant with a secondary closed loop refrigeration cycle using a second refrigerant, to maximize the available cooling power of the first refrigerant.
The cooling power is an important design parameter of a cryosurgical instrument. With greater cooling power, more rapid temperature decreases occur, and lower temperatures can be maintained at the probe tip during freezing. This ultimately leads to greater tissue destruction. The power of a J-T cryosurgical device is a function of the enthalpy difference of the primary refrigerant and the mass flow rate. Pre-cooling certain refrigerants will increase the enthalpy difference available for cooling power. In addition, pre-cooling will increase the average mass flow rate by making the gas more dense.
Pre-cooling has two other important ramifications. First, it reduces the size of primary-to-primary heat exchangers used in the probe or catheter. In the miniature environments envisioned for the used of this apparatus, severe size limitations will be placed upon the heat exchangers used. Second, J-T cryosurgical devices require fixed size expansion elements, which can become partially or totally blocked by contaminants such as water or oil. This limits flow rate and decreases cooling power. Pre-cooling allows these contaminants to be cold-filtered and removed from circulation prior to reaching the expansion element.
The primary closed loop refrigeration system has a primary loop compressor for compressing a primary refrigerant to a pressure up to 350 psia. An example of a suitable primary refrigerant is SUVA-95 made by DuPont Fluoroproducts, Chestnut Run Plaza, Wilmington, Delaware. Primary refrigerant mixtures of three or more constituent gases may also be used, as disclosed in parent application 08/698,004. The method and apparatus of the present invention can be used equally well in a rigid hand held cryoprobe, or in a catheter.
The high pressure primary refrigerant from the primary compressor is fed into a high pressure supply tube, such as an inner tube of a coaxial dual lumen tube leading to the handle of a cryoprobe, or to the proximal end of a flexible catheter. The dual lumen tube feeds the high pressure refrigerant into the inlet port at the proximal end of a miniature primary-to-secondary heat exchanger, which can be located in the handle of the cryoprobe, or at the proximal end of a flexible catheter. The high pressure primary refrigerant passes through the high pressure passageway within the primary-to-secondary heat exchanger and exits through a port at the distal end of the primary-to-secondary heat exchanger. If required, a primary-to-primary heat exchanger can be interposed between the compressor and the primary-to-secondary heat exchanger.
The primary-to-secondary heat exchanger is part of the secondary closed loop refrigeration system which has a secondary compressor and a secondary expansion element, in addition to the primary-to-secondary heat exchanger. The secondary compressor unit compresses and condenses a secondary refrigerant, different from the primary refrigerant, to a pressure which can be relatively higher than that used in the primary loop. A suitable secondary refrigerant is a 50/50 mix of difluoromethane and pentafluoroethane, or a 50/50 mix of pentafluoroethane and 1,1,1 trifluoroethane. Since the secondary loop does not flow into the probe or catheter, a higher pressure can be used safely. After passing through the condenser, the secondary refrigerant liquid passes through the secondary expansion element, in which the secondary refrigerant liquid evaporates and expands to a lower temperature.
The low pressure secondary refrigerant then passes through a low pressure secondary passageway in the primary-to-secondary heat exchanger and returns to the secondary compressor.
The outlet of the high pressure primary passageway of the primary-to-secondary heat exchanger can be connected to the inlet of a high pressure passageway in a miniature primary-to-primary heat exchanger located in the probe handle or at the proximal end of the catheter. The high pressure primary refrigerant passes through the high pressure passageway within the miniature primary-to-primary heat exchanger and exits through a port at the distal end of the heat exchanger. The high pressure passageway is then connected to the inlet of the primary Joule-Thomson expansion element located in the cold tip, in which the primary refrigerant is expanded to a lower pressure and a lower temperature, which can be as low as 148 K. If the primary-to-secondary heat exchanger has sufficient capacity, it may not be necessary to incorporate the first primary-to-primary heat exchanger.
The primary refrigerant exiting the primary Joule-Thomson expansion element is exposed to the inner surface of a heat transfer element mounted in the wall of an outer tube which is coaxial with the inner tube. The expanded primary refrigerant cools the heat transfer element to a lower temperature and then returns through the low pressure return passageway of the miniature primary-to-primary heat exchanger. This cools the primary high pressure refrigerant from its precooled temperature to a lower temperature. From the low pressure outlet of the miniature primary-to-primary heat exchanger, the low pressure expanded primary refrigerant flows to the first primary-to-primary heat exchanger, if present, or directly into the lumen of the outer coaxial tube, outside the inner high pressure tube, to return to the primary compressor.
Both the primary-to-secondary heat exchanger and the primary-to-primary heat exchanger can be coiled tube heat exchangers or finned tube heat exchangers. The primary-to-primary heat exchanger can be a coiled coaxial tube, with the inner lumen being the high pressure passageway and the outer lumen being the low pressure passageway. The secondary passageways of the primary-to-secondary heat exchanger can be a coiled coaxial tube, with the outer lumen being the low pressure secondary passageway. Attached to the side of this outer tube, in a parallel arrangement, is a tube which forms the high pressure primary passageway. The high pressure primary tube can have a plurality of inner tubes, which can be nested and placed in contact with the outer tube for improved heat exchange. High pressure primary refrigerant flows in all of the nested tubes and in the interstitial spaces between the nested tubes and the outer tube. The secondary tube and the primary tube can be formed of metal and soldered together.
Alternatively, the primary-to-secondary heat exchanger can consist of a coiled finned tube for the primary refrigerant, within a can into which the secondary refrigerant is expanded.
The miniature primary-to-primary heat exchanger can be a single coiled finned tube surrounded by a low pressure return passageway.
The primary Joule-Thomson expansion element can be a sintered metal plug made by sintering a plurality of metal beads into a metal cup, to provide the required pressure drop. Alternatively, the expansion element can be a properly sized orifice or some other type of restriction. The two different stages of the sintered plug expansion element, if present, can utilize different sizes of beads, different cross sectional areas, and different packing densities. The heat transfer element can take the optimum shape for matching the object or tissue to be cooled. For example, a metal plug can be installed in the tip of the outer tube or catheter, for applying cooling through the extreme distal tip of the catheter. Alternatively, a relatively narrow metal strip can be mounted in a side wall of the catheter, near the distal tip, for applying cooling to a narrow strip of tissue.
The novel features of this invention, as well as the invention itself, will be best understood from the attached drawings, taken along with the following description, in which similar reference characters refer to similar parts, and in which: